1. Technical Field
The invention is related to the formation of Vertical Cavity Laser (VCL) structures, and more particularly to a process for forming a VCL structure using an intermixing technique that includes a high temperature annealing operation.
2. Background Art
A typical VCL structure is formed from a layered semiconductor material, such as the material depicted schematically in FIG. 1. As can be seen, the depicted structure includes an oxidation region consisting of a thin AlAs layer and an adjacent 90% AlGaAs layers. In addition, there is usually a p-type distributed Bragg reflector (DBR) formed of alternating layers of GaAs and 90% AlGaAs overlying the oxidation region. Similarly, an n-type DBR is typically formed of alternating layers of GaAs and 90% AlGaAs, and underlies the oxidation region. The p-type DBR (or p-mirror) is usually created by doping the region via conventional methods with a p-type dopant such as beryllium (Be). The n-type DBR (or n-mirror) is similarly doped with a n-type dopant such as silicon (Si). The layered semiconductor material is typically etched to form a circular pillar or mesa consisting of the entire p-mirror and oxidation regions, and may extend partially into the n-mirror region.
In general, the oxidation region is formed from a series of layers having a graded content of an oxidizable material. In the exemplary structure depicted in FIG. 1, this oxidizable material is aluminum. The aforementioned grading results in the middle layer in the series of layers making up the oxidation region having the greatest overall concentration of the oxidizable material, while the layers overlying and underlying the middle layer comprise overall concentrations of the oxidizable material that are progressively less the further the layer is from the middle layer. In the exemplary structure depicted in FIG. 1, a layer of AlAs acts as the middle layer with adjacent layers of 90% AlGaAs constituting the overlying and underlying layer of having less of the oxidizable material (i.e., aluminum). It is noted that the xe2x80x9c90%xe2x80x9d designation indicates that there is only 90 percent of the amount of aluminum in a 90% AlGaAs, than in a AlAs layer. While not depicted in FIG. 1, the oxidation region could have additional outlying layers that contain even less aluminum. Further, the middle layer need not be pure AlAs. For example, in reference [1] the middle layer in the oxidation region of a tapered oxide apertured VCL structure was a layer of AlGaAs having an aluminum concentration greater than about 95%. The immediately adjacent oxidation region layers in the VCL structure of reference [1] were also AlGaAs, although with an aluminum concentration that is less than the middle layer but greater than about 70%.
The graded layer configuration of the oxidation region is of particular significance. While, a VCL structure can be formed with non-tapered oxide apertures, and sometimes is, the aforementioned grading will produce such a tapered shape [2]. A tapered oxide aperture more closely approximates an ideal lens and so, inter alia, lowers optical scattering losses and increases the performance of a small diameter VCL device. As such a tapered oxide aperture, while not required, is preferred. The tapered oxide aperture is created in the aforementioned oxidation region by taking advantage of the rapid reduction of the oxidation rate with lower Al content in the semiconductor layers. For example, in the exemplary VCL structure depicted in FIG. 1, the AlAs layer oxidizes at a rate approximately 10 faster than the adjacent 90% AlGaAs layer. Thus, when the previously formed mesa is subjected to an oxidation process, the thin AlAs layers oxidizes inwardly faster than the 90% AlGaAs layer producing the desired tapered oxide aperture as shown in FIG. 2(b).
Vast improvements in VCL efficiencies at low-output powers have been made possible by reducing the device size [3]. However, a persistent problem with current tapered, oxide-apertured, VCLs is that if the devices are made too small, the efficiency declines due to a current leak path caused by lateral carrier diffusion away from the center of the active region of the VCL. For example, the threshold current for a 6 xcexcm diameter VCL device is twice the value expected from extrapolation of the broad-area threshold current density. This current leakage path is due to the aforementioned lateral carrier diffusion away from the center of the active region, the effect of which becomes even worse at smaller diameters.
It is noted that in the preceding paragraphs, as well as in the remainder of this specification, the description refers to various individual publications identified by a numeric designator contained within a pair of brackets. For example, such a reference may be identified by reciting, xe2x80x9creference [1]xe2x80x9d or simply xe2x80x9c[1]xe2x80x9d. A listing of the publications corresponding to each designator can be found at the end of the Detailed Description section.
The present invention is directed at a process of forming a Vertical Cavity Laser (VCL) structure that overcomes the lateral diffusion problems of prior processes. Specifically, it has been discovered that the lateral diffusion phenomenon can be ameliorated by the use of standard intermixing techniques. However, these intermixing techniques require the VCL structure to undergo a high temperature annealing operation (e.g., 920 degrees C. for 180 s). It was found that the high temperature degraded the VCL structure considerably. For example, structures using beryllium (Be) as the p-type dopant experienced a high diffusion rate of the Be which caused an extensive redistribution of the dopant profilexe2x80x94so much so that the VCL would not lase. It is believe other p-type dopants, such as Zinc (Zn) or Magnesium (Mg), would cause similar or worse diffusion problems. In addition, the presence of the aforementioned oxide in the VCL structure during the annealing process caused serious structural degradation. Specifically, voids formed in the GaAs DBR layers which introduced excess scattering losses and a concomitant decrease in the efficiency of the VCL device. In addition, a noticeable downward bending of the layers along the circumference of the device was induced by the annealing process.
The present invention provides a solution to the degrading effects of the aforementioned high temperature annealing process on VCL structures. First, the p-type dopant is restricted to a low diffusivity dopant such as carbon, thus eliminating the diffusion problems associated with Be (or Zn, or Mg). Further, the oxide created to act as the aperture of the VCL is selectively removed leaving behind an air gap having the desired tapered shape. This oxide removal was accomplished by using a wet chemical approach employing a strong alkaline solution (e.g., a solution including KOH or NaOH) having a pH exceeding about 12-13. It was found that none of the above-described degradation occurred during the annealing of a VCL structure using carbon as the p-type dopant and air gap apertures. In addition, it was determined that the annealed, air gap apertured, VCL provided the same low optical loss properties previously attributed only to an un-annealed, oxide-apertured, VCLxe2x80x94but without sacrificing efficiency due to lateral carrier diffusion.